Transfers of Assets between Level 1 and Level 2 (Detail) - First Quarter - USD ($) $ in Millions	Mar. 31, 2018	Mar. 31, 2017
Transfers Between Level 1 and Level 2
Level 1 from Level 2	$ 5,843.5	$ 7,475.4
Level 2 from Level 1	1,015.3	1,312.8
United States Government and Government Agencies and Authorities
Transfers Between Level 1 and Level 2
Level 1 from Level 2	40.1	54.5
Level 2 from Level 1	0.0	180.0
Public Utilities
Transfers Between Level 1 and Level 2
Level 1 from Level 2	528.5	1,103.9
Level 2 from Level 1	62.0	57.5
All Other Corporate Bonds
Transfers Between Level 1 and Level 2
Level 1 from Level 2	5,274.9	6,317.0
Level 2 from Level 1	953.3	1,075.3
Equity Securities
Transfers Between Level 1 and Level 2
Level 1 from Level 2	24.2	0.0
Level 2 from Level 1	$ 14.9	$ 0.0